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  | isbn    = 0891816607

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Marine seismic data acquisition techniques and capabilities have advanced at a rapid pace over the past few years. Today the typical marine seismic source is much more powerful, efficient, and effective due to improvements in towing technology, air gun array design, and the air guns themselves. Towing two parallel streamer cables in three-dimensional surveys is now the rule rather than the exception, and recording between 240 and 480 channels of seismic data is common. The ability to determine the position of everything in the water on a shot-by-shot basis has also improved significantly.

Marine seismic data acquisition techniques and capabilities have advanced at a rapid pace over the past few years. Today the typical marine seismic source is much more powerful, efficient, and effective due to improvements in towing technology, air gun array design, and the air guns themselves. Towing two parallel streamer cables in three-dimensional surveys is now the rule rather than the exception, and recording between 240 and 480 channels of [[seismic data]] is common. The ability to determine the position of everything in the water on a shot-by-shot basis has also improved significantly.

==Source==

==Source==

The length of a streamer cable ranges from 3000 to [[depth::6000 m]]. Shorter cables can be deployed in areas with obstructions to improve control and thereby avoid damage. The length of cable required for a job depends on the objectives of the survey. In general, the deeper the objective, the longer the cable should be. A rule of thumb is that ''x'', the length of the cable, should equal z, the depth of the objective.

The length of a streamer cable ranges from 3000 to [[depth::6000 m]]. Shorter cables can be deployed in areas with obstructions to improve control and thereby avoid damage. The length of cable required for a job depends on the objectives of the survey. In general, the deeper the objective, the longer the cable should be. A rule of thumb is that ''x'', the length of the cable, should equal z, the depth of the objective.

Some streamer cables contain electronics to convert the analog signal from the hydrophones to a digital form. These are called ''digital cables'', while the traditional streamer cables are called ''analog cables''. In an analog cable, there must be a pair of wires for each group. As the number of groups increases, the number and weight of the copper conductors increase, which necessitates that the diameter of cable gets larger and larger. There are analog cables with as many as 240 channels<ref name=pt07r4>Carlini, A., and A. Mazzotti, 1989, Optimized receiver array simulation based upon resolution constraints: Geophysical Prospecting, v. 37, p. 607–621., 10., 1111/gpr., 1989., 37., issue-6</ref>. Digital cables, however, can multiplex data onto one or a few conductors, so there is not a weight problem due to conductors. Some digital cables have as many as 1000 channels. Analog cables may be more susceptible to electrical leakage, but a well-maintained analog cable can gather data equal in quality to that gathered using a digital cable.

Some streamer cables contain electronics to convert the analog signal from the hydrophones to a digital form. These are called ''digital cables'', while the traditional streamer cables are called ''analog cables''. In an analog cable, there must be a pair of wires for each group. As the number of groups increases, the number and weight of the copper conductors increase, which necessitates that the diameter of cable gets larger and larger. There are analog cables with as many as 240 channels<ref name=pt07r4>Carlini, A., and A. Mazzotti, 1989, Optimized receiver array simulation based upon resolution constraints: Geophysical Prospecting, v. 37, p. 607–621.</ref>. Digital cables, however, can multiplex data onto one or a few conductors, so there is not a weight problem due to conductors. Some digital cables have as many as 1000 channels. Analog cables may be more susceptible to electrical leakage, but a well-maintained analog cable can gather data equal in quality to that gathered using a digital cable.

In 3-D seismic surveys, the seismic boat commonly tows two and even three parallel cables spaced laterally apart so that two or three seismic lines are collected per traverse of the vessel. In a two-cable application with a single source, two seismic lines spaced ''L'' meters apart in the subsurface will be acquired if the streamer cables are spaced 2''L'' apart.

In 3-D seismic surveys, the seismic boat commonly tows two and even three parallel cables spaced laterally apart so that two or three seismic lines are collected per traverse of the vessel. In a two-cable application with a single source, two seismic lines spaced ''L'' meters apart in the subsurface will be acquired if the streamer cables are spaced 2''L'' apart.

==Navigation==

==Navigation==

It is essential that the precise location of the survey be known, and this is where navigation is important. The work horse of the industry, certainly in the Gulf of Mexico, is the shore-based navigation system ''Syledis''. In the Gulf and in other areas, base stations have been placed on production platforms to extend the range of coverage. Another system, Star-Fix, uses commercial satellites and a shore-based network of stations to determine the position of the satellites precisely. Given this information, the position of the vessel can be calculated. Another satellite system is the Global Positioning System (GPS), which is now being installed by the U.S. government. It is still unclear whether its capabilities will be intentionally degraded for commercial applications. GPS (or differential GPS) has the potential to make all other navigation systems obsolete.

It is essential that the precise location of the survey be known, and this is where navigation is important. The work horse of the industry, certainly in the [[Gulf of Mexico]], is the shore-based navigation system ''Syledis''. In the Gulf and in other areas, base stations have been placed on production platforms to extend the range of coverage. Another system, Star-Fix, uses commercial satellites and a shore-based network of stations to determine the position of the satellites precisely. Given this information, the position of the vessel can be calculated. Another satellite system is the Global Positioning System (GPS), which is now being installed by the U.S. government. It is still unclear whether its capabilities will be intentionally degraded for commercial applications. GPS (or differential GPS) has the potential to make all other navigation systems obsolete.

The advent of 3-D seismic has increased the demand for accuracy in positioning. One needs to know within a few meters the position of the source and the position of each detector group for every seismic shot. Underwater, acoustical systems and/or laser measurement through the air are used to determine the positions of things in the water near the vessel. Extremely accurate magnetic compasses are attached along the streamer cable, and sometimes a navigation receiver is placed in the tail buoy at the end of the streamer cable. Accuracies of a few meters are desirable.

The advent of 3-D seismic has increased the demand for accuracy in positioning. One needs to know within a few meters the position of the source and the position of each detector group for every seismic shot. Underwater, acoustical systems and/or laser measurement through the air are used to determine the positions of things in the water near the vessel. Extremely accurate magnetic compasses are attached along the streamer cable, and sometimes a navigation receiver is placed in the tail buoy at the end of the streamer cable. Accuracies of a few meters are desirable.

==Quality control==

==Quality control==

It is essential to have stringent but fair quality control (QC) specifications on each and every aspect of the marine seismic data acquisition system. Air guns will break. The QC specifications must address how many air guns can fail before acquisition must be terminated for repair. Things will go wrong with the streamer cable. How many channels can go bad before the cable must be repaired? Sometimes there are other seismic crews in the area. How much interference noise<ref name=pt07r36>Lynn, W., M. Doyle, K. Larner, and R. Marschall, 1987, Experimental investigation of interference from other seismic crews: Geophysics, v. 52, p. 1501–1524., 10., 1190/1., 1442268</ref> can be tolerated?

It is essential to have stringent but fair quality control (QC) specifications on each and every aspect of the marine seismic data acquisition system. Air guns will break. The QC specifications must address how many air guns can fail before acquisition must be terminated for repair. Things will go wrong with the streamer cable. How many channels can go bad before the cable must be repaired? Sometimes there are other seismic crews in the area. How much interference noise<ref name=pt07r36>Lynn, W., M. Doyle, K. Larner, and R. Marschall, 1987, Experimental investigation of interference from other seismic crews: Geophysics, v. 52, p. 1501–1524</ref> can be tolerated?

The details of this very important subject are beyond the scope of this note, but in my opinion, it is almost as easy to keep things running at 90 to 100% capacity as it is to keep things running at 70 to 80%, so it makes sense to demand high performance.

The details of this very important subject are beyond the scope of this note, but in my opinion, it is almost as easy to keep things running at 90 to 100% capacity as it is to keep things running at 70 to 80%, so it makes sense to demand high performance.

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]

[[Category:Seismic data acquisition]]

[[Category:Seismic data acquisition]]